1. Field of the Invention
The present invention relates to an A.C. current control system for a controlled A.C. power supply device destined to supply a controlled A.C. current of a predetermined waveform pattern to an A.C. load.
2. Description of the Prior Art
For the controlled A.C. power supply device of the above type, there have been hitherto known a cyclo converter and a pulse width modulation (PWM) inverter in which semiconductor switches such as thyristors are employed. In order to have a better understanding of the present invention, description will be first made on a typical example of the heretofore known A.C. current control system for a power supply device shown in FIG. 1 of the accompanying drawings. Referring to this figure, reference numeral (100) generally denotes a control circuit for controlling A.C. current supplied from a power supply device (200) to a load (300) such as an electric motor M having a field winding (320) adapted to be energized from a field current control circuit (400). The control circuit (100) comprises a voltage controller (101), a voltage-to-frequency converter (102), a waveform pattern generator (103), current controllers (111) to (113), a gate signal generator (107), a voltage detector (105) and a rectifier circuit (104). Reference symbol V.sub.s designates a reference voltage, I.sub.s represents a reference current, I.sub.u, I.sub.v and I.sub.w represent individual phase currents, F.sub.s represents a speed command signal, P.sub.s represents a command pulse train corresponding to the input speed command signal F.sub.s, I.sub.su, I.sub.sv and I.sub.sw represent reference phase currents, I.sub.fu, I.sub.fv and I.sub.fw represent feedback currents for the respective phases, and V.sub.su, V.sub.sv and V.sub.sw represent power supply command or control voltages for the respective phases.
The control circuit (100) has various functions for controlling electric power supplied to the load (300). The phase feed voltages are detected by the voltage detector (105) and supplied to the voltage rectifier (104), the output from which is a rectified voltage V.sub.d adapted to be utilized as a feedback control voltage. The voltage controller (101) receives the feedback voltage V.sub.d and the reference voltage V.sub.s thereby to arithmetically determine any deviation of the feedback voltage V.sub.d from the reference voltage V.sub.s and produce the reference current I.sub.s. On the other hand, the speed command voltage F.sub.s is converted into the command pulse train P.sub.s through the voltage-frequency converter (102). The pattern generator (103) serves to generate reference phase currents I.sub.su, I.sub.sv and I.sub.sw of a predetermined waveform pattern on the basis of the reference current I.sub.s and the command pulse train P.sub.s. Such waveform pattern may be of sine, trapezoidal or the like shape. The feedback current signals I.sub.fu, I.sub.fv and I.sub.fw are detected by appropriate current detectors (114), (115) and (116) such as current transformers and supplied to the current controllers (111), (112) and (113) to be compared with the reference current signals I.sub.su, I.sub.sv and I.sub.sw, respectively. The output voltage signals V.sub.su, V.sub.sv and V.sub.sw from the controllers (111), (112) and (113) which represent deviations of the feedback current signals I.sub.fu, I.sub.fv and I.sub.fw from the respective reference current signals L.sub.su, I.sub.sv and I.sub.fv constitute the voltage control signals V.sub.su, V.sub.sv and V.sub.sw for controlling the phase feed voltages supplied from the power supply device (200) to the load (300). The gate control circuit (107) responds to the voltage command signals V.sub.su, V.sub.sv and V.sub.sw to generate corresponding gate control signals which are applied to gate electrodes of the semiconductor controlled switches such as thyristors constituting the power supply device (200) for controlling the on-off operations of the switches. In this manner, the A.C. power which is controlled so as to be substantially proportional to the voltage command signal V.sub.s is supplied to the load (300) from the A.C. power supply device (200). By virtue of the provision of such closed control loop, the A.C. current supplied to the load (300) can be controlled by the phase feed voltage so as to follow the reference current I.sub.s of the predetermined waveform pattern.
It is to be noted that the load (300) adapted to be fed from the current supply and control system such as described above is in many cases constituted by an inductive load or a load capable of producing an internal electromotive force. The latter load may be exemplified by polyphase alternating machines such as inductive motors, synchronous motors or the like.
FIG. 2 illustrates schematically in a functional transfer block diagram the principle of the current control loop of the hitherto known control system described above. The reference numeral (200) designates the A.C. power supply device, while the load is designated by the reference numeral (300) as is in the case of FIG. 1. Reference symbol I.sub.s represents the reference current, I.sub.a represents the phase current, and V.sub.a represents the phase feed voltage. The symbol V.sub.s represents the phase voltage command signal.
It is assumed that the load (300) is constituted by an electric motor producing an internal electromotive force. In other words, the motor is inherently imparted with an current control loop having a feedback control quantity e.sub.f. As described hereinbefore, the voltage command signal V.sub.s and hence the feed voltage V.sub.a supplied to the load (300) are determined by deviation of the actual feed current I.sub.a from the reference current I.sub.s. However, since the current to be controlled will respond to the difference between the feed voltage V.sub.a and the internal electromotive force e.sub.f, the deviation or the command voltage V.sub.s derived therefrom is required to have an amplitude increased for a magnitude corresponding to the internal electromotive force e.sub.f. Accordingly, in the case of the load such as the electric motor exhibiting intrinsically the internal electromotive force, the current control deviation (instantaneous deviation including absolute deviation and phase deviation) will tend to be increased. In particular, in the case of a variable speed polyphase A.C. motor, the current control deviation is excessively increased in a high speed region due to the remarkable increase in the electromotive force, making it impossible or at least difficult to control the current waveform in the desired sine or trapezoidal form, whereby the range in which the motor speed can be varied is disadvantageously restricted. Further, the hitherto known power supply control system has been disadvantageous in that the torque of a required magnitude cannot be attained in the high speed range of the motor. In general, the conventional power supply control system for a varying load is undesirably susceptible to undergoing great deviations in respect of the current waveform to be controlled.